Mercury Porosimetry for Characterization of Specialty Papers
Understanding paper properties, such as pore size and pore volume, and how they change upon coating sheds light on the printability and ink spreading and penetration properties of paper. Because the pores are in the micron size range, mercury porosimetry, performed with the Anton Paar PoreMaster series, is well-suited for paper structural characterization.
Introduction Mercury porosimetry is a useful technique for the characterization of a wide variety of porous coated and uncoated papers because it can provide structural information such as:
- pore size distribution
- pore volume
- percent porosity
- bulk density
The mercury intrusion technique is based on the fact that a non-wetting fluid, such as mercury, will not penetrate the pores (or voids) of a porous material unless acted upon by a pressure large enough to cause intrusion. The equation governing this behavior and the working equation in mercury porosimetry is the Washburn equation:
d=-4γcosθ / P
where d is the pore diameter, γ is the surface tension of the intrusion fluid, θ is the contact angle formed by the intrusion fluid on the solid, and P is the pressure applied to the intrusion fluid. The surface tension of mercury is taken as a constant and the contact angle formed by mercury on a wide array of solids has been shown to be approximately 140 degrees . The Washburn equation predicts the pressure required to intrude a non-wetting fluid such as mercury into a pore of a given diameter. The mercury intrusion technique uses this relationship to derive pore size information and uses the volume change of intruded mercury over small pressure (diameter) intervals to derive pore size distribution information.
Mercury porosimetry can provide valuable information on specialty papers used in printing applications. Properties such as ink spreading, penetration, and adsorption (printability) correlate strongly with pore distribution and pore volume of the paper and the paper coating. Because the aim of many paper producers and converters is to optimize these properties for a particular printing application, mercury porosimetry can aid in product development.
1. S. Lowell, J.E. Shields, M.A. Thomas, M. Thommes. Characterization of Porous Solids and Powders, Springer, 2006.
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